1. Field of the Invention
The present application relates to a fuel injector particularly adapted for use with compressed natural gas. The injector includes a magnetic pole flux director, which reduces flux leakage and improves the effectiveness of the magnetic circuit.
2. Description of the Related Art
Compressed natural gas (hereinafter sometimes referred to as xe2x80x9cCNGxe2x80x9d) is becoming a common automotive fuel for commercial fleet vehicles and residential customers. In vehicles, the CNG is delivered to the engine in precise amounts through gas injectors, hereinafter referred to as xe2x80x9cCNG injectorsxe2x80x9d. The CNG injector is required to deliver a precise amount of fuel per injection pulse and maintain this accuracy over the life of the injector. In order to maintain this level of performance for a CNG injector, certain strategies are required to optimize the combustion of the fuel.
The CNG (Compressed Natural Gas) injector is required to open and close very quickly to promote efficient fuel consumption. In order to accomplish this objective effectively the magnetic circuit utilized to open the valve needle must produce a magnetic fieldxe2x80x94or fluxxe2x80x94relatively quickly across the working gap between the fuel inlet connector and the armature. The present CNG injector has a magnetic circuit consisting of an inlet connector, armature, valve body shell, housing and a coil. When energized, the coil produces a magnetic field, which is conducted through the magnetic circuit. The flux is conducted through the components and creates an attractive force at the working gap, which force causes upward movement of the armature, with consequent upward movement of the valve needle to open the injector valve.
It is known that the magnetic field will be conducted through any surrounding path of least reluctance because the property of reluctance in a magnetic circuit is similar to resistance in an electrical circuit. For example, ferrous steels have a reluctance considerably less than the reluctance of air. If the magnetic circuit has poor material, poor design, or a working gap that is too large, the magnetic field will xe2x80x9cleakxe2x80x9d out of the intended path and complete the circuit using the path of least reluctance. Moreover, the magnetic force is inversely proportional to the square of the distance through which it must act. Accordingly, avoidance of leakage of magnetic flux is significant and desirable.
This magnetic leakage reduces the effectiveness of the field that is generated and requires that more fields be created to reach the designed level of performance. In most injector designs, it is necessary to have a smaller outer diameter for the armature than the outer diameter of the pole piece. This is driven by constraints to reduce the outer diameter of the injector and improve performance. To achieve the smaller outer diameter, the magnetic cross section must be reduced. One of the easiest components to reduce is the diametrical cross section of the coil. It can be made longer and thinner and wound to a number of turns and resistance which is acceptable. This strategy tends to increase the leakage through the coil by bringing the housing closer to the inlet connector. As the reluctance of any portion of the circuit, usually the working gap portion due to the increased reluctance of air, becomes greater than a direct path from the inlet connector to the housing, leakage occurs. We have invented a fuel injector, which directs the magnetic flux to the desired locations with increased intensity and reduced leakage.
The invention relates to an electromagnetically operable fuel injector for a fuel injection system of an internal combustion engine, the injector having a generally longitudinal axis, which comprises, a ferromagnetic core, a magnetic coil at least partially surrounding the ferromagnetic core, an armature magnetically coupled to the magnetic coil and being movably responsive to the magnetic coil, the armature actuating a valve closing element which interacts with a fixed valve seat of a fuel valve and being movable away from the fixed valve seat when the magnetic coil is excited. The armature has a generally elongated shape and a generally central opening for reception of fuel from a fuel inlet connector positioned adjacent thereto. A fuel inlet connector extends generally longitudinally of the injector and is positioned adjacent the armature and defines a central path for fuel to enter the inlet connector and to be directed toward the armature for further passage toward the fixed valve seat. The fuel inlet connector has a lowermost end portion spaced above the armature and facing the fuel inlet connector to define a working gap through which the armature is movable to open and close the valve. The lowermost end portion of the fuel inlet connector has a transverse cross-sectional dimension corresponding to the transverse dimension of an upper end face of the armature to define a substantially direct magnetic flux path between the armature and the fuel inlet connector.
Preferably the fuel inlet connector has a lowermost surface spaced adjacent and above the armature to define a working gap through which the armature is movable, the lowermost end portion of the fuel inlet connector defining with the armature and the valve body, a direct path for magnetic flux. The fuel inlet connector has a chamfered configuration along the lower end portion on the outer side thereof so as to reduce the thickness of the fuel inlet connector whereby the reduced thickness portion is in direct face-to-face relation with the opposed upper surface portion of the armature thereby providing a ferromagnetic metal-to-metal path for magnetic flux lines generated by the coil and thereby reducing leakage of the flux lines outside of the ferromagnetic metal-to-metal path.
The lower end portion of the fuel inlet connector is positioned adjacent the armature and has a generally chamfered configuration along the lowermost outer end thereof to reduce the effective surface area of the lowermost face of the fuel inlet connector to substantially match the upper end face of said armature. The generally chamfered portion of the fuel inlet connector is preferably arcuate in cross-section, and a valve needle is adapted for selective engagement and disengagement with the fixed valve seat.
In the preferred embodiment fuel inlet connector and the armature are adapted to permit a first flow path of gaseous fuel between the armature and the magnetic coil as part of a path leading to the fuel valve. The armature defines at least one first fuel flow aperture extending through a wall portion thereof to define a second flow path of gaseous fuel as part of a path leading to the fuel valve. The armature defines at least one-second aperture in a wall portion to define a third flow path of gaseous fuel as part of a path leading to said fuel valve, the second aperture being oriented at a generally acute angle with respect to the longitudinal axis.
The electromagnetically operable fuel injector further comprises a valve body positioned downstream of the armature and having at least one aperture in a wall portion thereof for reception of fuel from at least two of the flow paths of gaseous fuel from the armature and the fuel inlet connector. The fuel inlet connector is positioned above the armature and is spaced from the armature by the working gap, the fuel inlet connector defining a central opening for directing fuel toward the armature and the fixed valve seat. The fuel inlet connector further includes a fuel filter at an upper end portion thereof for filtering fuel.